The invention relates to a method for constructing a linear or punctiform structure on a support, especially for constructing strip-like electrically conducting contacts on a semi-conductor element such as a solar cell by applying an electrically conducting paste-like substance containing a solvent adhering to the support and subsequent hardening of the substance.
In manufacturing electronic structural elements the application of fine electrically conducting structures primarily takes place through physical or chemical gas phase precipitation, epitactic methods using masks or possibly with additional laser support. These techniques permit the manufacture of very fine structures, even if these are hardly suited for an economical mass production for economic reasons.
Other methods for applying fine structures are, for example, screen printing, roller printing or tampon printing. Even if these methods appear attractive from economic perspectives, there is nonetheless the disadvantage that only a restricted resolution of the structures is attainable, so that using it is suited only for larger electronic components with coarser structures.
In manufacturing solar cells, there exists with reference to the side facing the irradiation the requirement of applying as fine electrically conducting structures as possible, which assure good electrical conductivity as well as good electrical contact to the solar cell, thus to the support. This requirement is imposed against the background that the surface facing the irradiation is to be shaded as little as possible. Nonetheless, in order to make possible good electrical conductivity with a high tapping, the corresponding leads must have a large cross-section. In order to meet these demands, contacts are frequently applied according to the state of the art using screen printing. But a precondition for this is that the support has a level surface upon which linear or strip-like contacts are applied. Epitactic methods are also applied.
With a further known method, depressions are introduced into a solar cell surface using a laser beam, which are subsequently filled with a conductive material using chemical precipitation methods (U.S. Pat. No. 4,726,850). But an appropriate method is quite expensive and therefore cost-engendering.
An electrically conducting paste is applied to a support in WO-A-91/24934, whereby the paste is polymerized and stabilized with UV light.
In order to construct a structure on a plasma screen, applying a substance with a binding agent decomposing by heat, which is then hardened by the action of temperature, is known according to U.S. Pat. No. 6,312,864.
U.S. Pat. No. 6,322,620 proposes the hardening of a substance on a support through thermal treatment.
Printing methods for applying structures on a solar cell can be gathered from the literature: “Screen-printed Rapid Thermal Processed (RTP) Selective Emitter Solar Cells Using a Single Diffusion Step,” 16th European Photovoltaic Solar Energy Conference, May 1-5, 2000, Glasgow, UK, p. 1087-1090 and “High Quality Screen-printed and Fired-through Silicon Nitride Rear Contacts for Bifacial Silicon Solar Cells,” 16th European Photovoltaic Solar Energy Conference, May 1-5, 2000, Glasgow, UK, p. 1332-1335.
A paste is known from JP-A-63268773 which is solvent-free and contains a noble metal powder, fritted glass and metal oxide as well as binding agents.
Printing methods have the basic disadvantage that no high layer thickness can be generated with a small line width. This leads to the disadvantage that wider lines or a larger number of these are necessary to attain the desired low resistance values. Consequently an undesirable shading takes place.
Finger writing systems, to the extent that these succeed in being used in structuring solar cells, are basically suited for fulfilling the requirements. But the line width depends strongly upon the interaction between the substance to be applied, as a rule in the form of a paste, and the surface properties of the support. Paste and support basically influence the spreading out of the former after application. Moreover, the structures applied in the form of linear strips manifest the tendency to run. The structure applied becomes partly coarse by this. Fine openings of the finger writing systems can be selected in order to avoid these disadvantages, owing to which, nonetheless, the risk increases that the strips to be applied tear. The high sensitivity of the process to the relevant process parameters therefore leads to the corresponding technique being used but seldom.